[Brijesh-Thakkar]

Addresses #19713.

ProtoCoder stores only the message class and finds its Parser lazily through getDefaultInstance(). For a generic T extends Message the type is erased to the com.google.protobuf.Message interface, which has no getDefaultInstance(). A coder for it can never decode, and the failure on the worker was a bare NoSuchMethodException: com.google.protobuf.Message.getDefaultInstance(), as reported in the issue.

The concrete type is not recoverable at decode time (the wire format carries no type information), so this PR does not try to make that coder work and does not change what succeeds or fails. It only makes the failure understandable: ProtoCoder.getParser() now throws an IllegalArgumentException (the same exception type as before, with the original NoSuchMethodException as the cause) saying why the parser cannot be found and to use a concrete generated message class, DynamicProtoCoder, or setCoder(). The existing DynamicMessage path is untouched.

Earlier attempts (#34194, #37005) added an instantiation fallback. That cannot work for an interface, and their tests did not fail before the change. The decode test here fails on master (old message, no guidance) and passes with the change.

Behaviour change: none beyond the error message. Encoding with a raw-Message coder still works and is covered by a test.

Testing:

• ./gradlew :sdks:java:extensions:protobuf:spotlessCheck :sdks:java:extensions:protobuf:test : 196 tests, all pass.
• testDecodeWithRawMessageClass fails on master (old NoSuchMethodException message), passes with the change. testEncodeWithRawMessageClass passes before and after.
• GCP IO (users of ProtoCoder): PubsubIOTest, PubsubUnboundedSinkTest, PubsubMessageWith*CoderTest, BigQueryIOStorageReadTest : 133 tests, all pass.
• CHANGES.md updated under Bugfixes.

Possible follow-up (for the maintainer to decide)

Coder inference (CoderRegistry) still returns a ProtoCoder<Message> silently for the raw Message type, so the failure only appears at decode time on the worker. A small follow-up would make ProtoCoderProvider.coderFor throw CannotProvideCoderException for it, so the failure moves to pipeline construction (test: testRegistryRejectsRawMessageType). I held it back because it is a behaviour change: a pipeline that relied on inference for an erased T extends Message and only ever encoded (never decoded, shuffled or checkpointed) works today and would then need an explicit setCoder(). No Beam code constructs a raw-Message coder (grep of main and test code), and the protobuf and GCP IO tests pass with it. It is ready as a separate local commit if wanted.

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📋 Review Summary

This PR improves the error message in ProtoCoder.getParser() when a NoSuchMethodException occurs while looking up getDefaultInstance() (such as when a ProtoCoder is created for the raw com.google.protobuf.Message interface due to type erasure). It preserves existing behavior, exception types, and causes while giving actionable guidance to users.

🔍 General Feedback

• The implementation is clean, minimal, and backwards-compatible (preserving IllegalArgumentException with the underlying NoSuchMethodException as its cause).
• Unit tests cover both the improved error message on decode and the existing behavior on encode.
• Regarding the follow-up mentioned in the PR description about rejecting raw Message.class in ProtoCoderProvider.coderFor: keeping this PR scoped to the decode error message is a safe choice, and rejecting raw Message / abstract classes during coder inference in ProtoCoderProvider would indeed be a good candidate for a separate follow-up discussion/PR.

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